1. Field of the Invention
This invention relates, in general, to position and velocity control systems and, more specifically, to controllers for changing the velocity of a movable member at selected locations along a path of movement.
2. Description of the Prior Art
In industrial applications, controllers have been devised for accurately controlling the travel extent and velocity of a member driven along a predetermined path. In such applications, it is necessary to start and stop the member at precise locations and to decelerate the member at appropriate points to assure that the desired stop point is not overshot. For this reason, virtually all prior art systems accelerate the driven member at selected rate until a maximum velocity of relative motion is attained. The relative velocity then remains constant until the selected location is reached along the path of movement at which time the moving member is decelerated until a minimum velocity is reached. Motion continues at the minimum velocity until the desired stop point is reached. The acceleration phase is used so that the driven member is gradually brought up to speed without undue strain on the system while overcoming the inertia of the driven member. The constant speed phase is selected in accordance with the maximum speed permissible for the particular system and operation to increase production efficiency. The deceleration phase is used to gradually stop the driven member and to prevent overshoot and thus, increase the accuracy of the system.
A typical controller for use in the above-described applications is disclosed in U.S. Pat. No. 4,031,442. This controller includes voltage sensitive means which provides a voltage indicative of the position of the driven member along its path of motion. The position indicative voltage is compared with reference voltages and the resulting comparison voltages are used to actuate switching circuitry to effect changes in the travel speed of the driven member and to control its start and stop positions.
While such a controller is effective in accurately controlling the travel velocity of a driven member along a path of movement, problems are encountered during initial start-up of the hydraulic or mechanical control apparatus which actually propells the driven member along its path of movement. As is well known, all hydraulic and mechanical apparatus go through a warm-up period after intial start-up or restart after a lengthy off time in the control fluids, lubricating oils, etc., undergo a temperature rise to normal operating temperatures. During the start up period, the response time of the hydraulic and/or mechanical control devices is slower than normal which causes the hydraulic and mechanical control apparatus to respond slowly to control signals for changing the velocity of the driven member during acceleration and deceleration periods.
The slower response time results in a longer deceleration period which causes the movable member to have considerable momentum a higher velocity than desirable when it reaches its final stop position. This results in the sudden stop of the driven member which could damage to the hydraulic and mechanical apparatus or the load carried by the driven member.
Thus, is would be desirable to provide a position and velocity control system which overcomes the problems of previously devised control system during initial start up or restart. It would also be desirable to provide a position and velocity control system which compensates for the slower response time of the hydraulic and/or mechanical drive mechanism during initial start up and restart. Finally, it would be desirable to provide a position and velocity control system which smoothly decelerates a driven member to a minimum velocity prior to reaching the final stop point along a path of movement despite the slower response time of the hydraulic and/or mechanical drive mechanism during initial start up and/or restart.